Field of the Invention
The present invention relates to an optical member having a high antireflective performance in a broad region including a visible region for a base material with a low refractive index, and a method for manufacturing the optical member.
Description of the Related Art
An antireflective structure having a fine structure corresponding to those equal to or smaller than wavelengths in a visible region is known to exhibit an antireflective performance in a broad wavelength region by forming a fine structure with an appropriate pitch and an appropriate height. As a method of forming the fine structure, a film in which fine particles having a particle diameter equal to or smaller than a wavelength are dispersed is known. A method of forming the fine structure by a pattern formation by a fine processing apparatus (such as an electron ray drawing apparatus, a laser interference exposure apparatus, a semiconductor exposure apparatus, an etching apparatus) enables control of a pitch and a height, and the method is also known to be able to form the fine structure having the good antireflective performance.
In addition to the method using the fine processing apparatus, it is known to obtain an antireflective effect by growing boehmite, which is an alumina-based compound, on the base material. In this method, a film is formed by a vacuum film formation or a liquid phase method (sol gel method), and then the film is dried or burned to produce a film of aluminium oxide. Subsequently, by treating the produced aluminium oxide film with water vapor or immersing the film in hot water, boehmite is produced on its surface layer to form a fine structure, followed by making an antireflective film (see Japanese Patent Application Laid-Open No. 9-202649).
In the method of producing the boehmite on the surface layer to form the fine structure by immersing in the hot water, aluminium oxide is eluted from the surface to form the fine structure. In Japanese Patent Application Laid-Open No. 9-202649, the antireflective film is formed by forming a layer supporting the fine structure between the substrate and the fine structure so that the substrate does not influence the fine structure. In the method of forming the antireflective film using the fine structure of the aluminium compound, a reflectance due to a vertical incidence and an oblique incidence is low, and the good antireflective performance can be obtained.
In Japanese Patent Application Laid-Open No. 2005-275372, it is described that when an antireflective film is produced by the liquid phase method (sol gel method), a refractive index of a layer supporting a fine structure is controlled by mixing another metal compound in a coating solution containing an aluminium compound.
However, the antireflective performance of the antireflective film using the fine structure of the aluminium compound disclosed in Japanese Patent Application Laid-Open No. 9-202649 sensitively changes due to changes of the refractive index attributed to the pitch and the height of the fine structure. Therefore, it is required to control not only the pitch and the height of the fine structure but also the refractive index of the layer supporting the fine structure according to the refractive index of the base material, and in some cases, it is also required to provide a film made of an inorganic material or an organic material for the purpose of regulating a refractive index difference between the base material and the aluminium oxide film having the fine structure.
In the method for manufacturing the antireflective film described in Japanese Patent Application Laid-Open No. 2005-275372, the metal compound to be mixed is limited to compounds of zirconia, silica, titania and zinc oxide, and thus, it is restricted that the refractive index of the layer supporting the fine structure is reduced and the pitch and the height of the fine structure are suitably controlled. In particular, it has been required to obtain the good antireflective performance for the base material having the low refractive index.